How to effectively UV unwrap a subdivided mesh?
The message is because the scales of your object are non uniform. That means that some axis has a different scale than the other (eg : (1, 2, 1)).

But, unwrap functions works on uniform scale. So, in order to keep the dimensions and unwrap with the good proportions, you need to apply the scale (with Ctrl+A then 'scale').

Here you can see the impact on the unwrapping :



By the way, it seems that your initial work on the UV map give few space for the main face of the mesh (it is in a small center part of the UVs).
You should unwrap the main faces as 'big surfaces' in order to have the best benefit of the texture.

Something like that, which is obtained automatically adding seams to each corner :